Parenting/Joining a helmet to a skeleton rig
The helmet consists of 2 elements msh_vents_topSide and msh_visor. You need to select both of them with SHIFT+click, then still with Shift select bone "head" (it's top most circle in "rig" rig), then press CTRL+P and choose Bone. This way whole helmet will be parented to head bone with weight 1 and no need for weight painting.